In spite of approximately 50 morphological and physiological criteria currently used in yeast systematics, DNA base composition studies have shown a wide intraspecific variation in moles per cent guanine plus cytosine (percent GC) in the nuclear DNA of a number of presently accepted yeast "species." Conversely, the indiscriminate use of a large number of such criteria has led to the description of many new "species" sometimes differing by only a single biochemial ability, for example, hydrolysis of a disaccharide. In several species of Saccharomyces and Debaryomyces, we have shown high degrees of DNA-DNA homology or renaturation ability between species separated on this basis. Such "species" may differ by only a single gene, or even by a single base pair in the case of point mutations. We are continuing to study strains of yeast species to determine variations in DNA base composition. As an example, a high proportion of the yeasts associated with necrotic tissue of cacti in the Sonoran desert were thought to be strains of Pichia membranaefaciens on the basis of current toxonomic criteria. On the basis of GC analyses, we have shown that none of these strains represent P. membranaefaciens, but that they consist of six novel separate species. New taxonomic parameters have been found to distinguish these species. We are also conducting DNA/DNA reannealing experiments between nuclear DNAs of what appear to be closely related species with similar base composition. For these experiments, we used DNA labeled in vitro with 125I. The duplexes are differentiated from unreacted single-stranded DNA by adsorption of hydroxylapatite. The results are expressed in percent relative binding of heterologous versus homologous DNA. Nineteen species of Kluyveromyces have been reduced to twelve by showing that four species were synonymous with K. marxianus and three with K. lactis.